The invention relates to a two-dimensional radiation detector for detecting radiation, such as X-ray, of an X-ray image-taking apparatus suitable for medical diagnosis, more particularly, a two-dimensional radiation detector of a solid operation system.
As known by Japanese Patent Publications (KOKAI) No. 4-212456, No. 4-212458 and No. 3-185863, a two-dimensional radiation detector is structured such that a semiconductor layer for sensing radiation, such as X-ray, or light converted from radiation to electric charges, and a switching element matrix formed of field effect transistors or the like are integrated, and switching elements are two-dimensionally scanned to obtain picture image signals. The structure thereof is shown in FIGS. 6 and 7.
In FIG. 6, reference numeral 1 is a semiconductor layer, one side surface of which is provided with a bias electrode 2 to be connected to a bias power source E and the other side surface of which is provided with a signal electrode 3 in a shape of matrix where detection elements, i.e. picture elements, are disposed two-dimensionally. Numeral 4 is a switching element matrix formed of switching elements 5, such as thin film transistors (hereinafter referred to as xe2x80x9cTFTxe2x80x9d), and the respective switching elements 5 are connected to the respective signal electrodes 3 of the semiconductor layer 1. The semiconductor layer 1 and switching element matrix 4 are produced by a thin film technique. Incidentally, in the drawings, numeral 6 represents condensers for storing charges, which are manufactured by the thin film technique in the same manner as in the semiconductor layer 1 and switching element matrix 5.
With the structure, in case radiation, for example X-ray, transmitted through a subject to be examined, enters the semiconductor layer 1 through the bias electrode 2, the X-ray is absorbed by the semiconductor layer 1 to produce a pair of electron-hole, i.e. charges.
The produced charges are subjected to a voltage shift by a voltage applied to the bias electrode 2 from the bias power source E and the charges are stored in the condenser 6. The charge amount stored in the condenser 6 varies corresponding to an incident X-ray dosage or amount to the semiconductor layer 1.
Switching lines 7 of the switching elements 5 for constituting the switching element matrix 4 are connected to a switching element drive circuit 8, and read-out lines 9 are connected to a multiplexer 11 through respective amplifiers 10, as shown in an equivalent circuit of FIG. 7. When the switching elements 5 are driven by the switching element drive circuit 8, the charges stored in the condensers 6 of the respective picture elements of one line, the switching elements 5 of which are turned on, are simultaneously outputted to the read-out lines 9. When the switching element drive circuit 8 sequentially drives the switching elements 5, the picture elements are two-dimensionally scanned and signals outputted to the read-out lines 9 are converted to picture image signals for the respective picture elements at the multiplexer 11 to be inputted to an A/D converter 12. Digital picture image signals of the respective picture elements are obtained at the A/D converter 12, and a two-dimensional X-ray picture image can be obtained by processing the digital picture image signals.
However, in the conventional two-dimensional radiation detector, there are problems as described below.
In case a large amount of radiation, such as X-ray, enters the semiconductor layer, a voltage applied to the condenser by charging of a stored charge is increased. The increase in the voltage applied to the condenser causes restriction of a dynamic range by saturation of the storage amount and a withstand voltage break of the switching element connected to the condenser.
In view of the above problems, an object of the invention is to provide a two-dimensional radiation detector, wherein in case a large amount of radiation exceeding a predetermined value enters any of detection elements, i.e. picture elements, a bias voltage to be applied to a bias electrode of a semiconductor layer is shut off to restrict a pair of electron-hole, i.e charges, produced in the semiconductor layer, a restriction of a dynamic range due to saturation of the storage amount of a condenser is removed, and a withstand voltage break of a switching element due to increase in a voltage applied to the condenser does not occur.
Another object of the invention is to provide a two-dimensional radiation detector having a wide dynamic range as stated above, wherein a picture image signal having an amount corresponding to an incident radiation or X-ray amount can be obtained.
A further object of the invention is to provide a two-dimensional radiation detector, wherein a pair of electron-hole, i.e. charges, produced in the semiconductor layer by incidence of radiation, is automatically restricted corresponding to an incident radiation amount to remove a restriction of a dynamic range by saturation of a storage amount due to increase in a voltage applied to a condenser, and a switching element connected to the condenser is not subjected to a withstand voltage break.
Further objects and advantages of the invention will be apparent from the following description of the invention.
In order to attain the above objects, according to a first aspect of the invention, a two-dimensional radiation detector includes a current detecting device for connecting a bias electrode of a semiconductor layer to a bias power source through a switch and detecting a current flowing through a circuit where the switch is interposed; and a control circuit for turning off the switch when a current value detected by the current detecting device reaches a threshold value.
According to a second aspect of the invention, the two-dimensional radiation detector of the first aspect of the invention further includes a compensation device for compensating an output signal q of each detection element, i.e picture element, at a switch-off time t2 to obtain a compensation signal Q according to the following equation:
Q=(t1xe2x88x92t0)/(t2xe2x88x92t0)*q
wherein to is a radiation start time; t1 is a radiation terminate time; t2 is a time when the switch is turned off in the control circuit; and q is an output signal of each picture element at the switch-off time t2.
With the structure, in the two-dimensional radiation detector according to the first aspect of the invention, when radiation, such as X-ray, enters the semiconductor layer, an electric current flows through the bias electrode of the semiconductor layer from a bias power source. The electric current flowing through the bias electrode is detected by the current detecting device to be provided to the control circuit. The control circuit compares the current value detected at the current detecting device and a predetermined threshold value, and when the detected current value exceeds the threshold value, the control circuit turns off the switch interposed between the bias electrode of the semiconductor layer and the bias power source.
Incidentally, the threshold value is set to a current value where the storage amount of the condenser is not saturated due to increase in a voltage applied to the condenser, and a switching element connected to the condenser is not subjected to a withstand voltage break.
Therefore, when a large amount of X-ray enters any of the detection elements, i.e. picture elements, of the semiconductor layer constituting the two-dimensional radiation detector and the current flowing through the bias electrode of the semiconductor layer through the switch from the bias power source reaches the threshold value, since the switch interposed between the bias electrode and the bias power source is turned off and application of the bias voltage to the bias electrode is shut off, increase in the voltage of the condenser is suppressed to thereby prevent the storage amount from being saturated and, at the same time, a withstand voltage break of the switching element connected to the condenser is prevented.
Also, in the two-dimensional radiation detector according to the second aspect of the invention, in case a large amount of X-ray enters any of the detection elements, i.e. picture elements, of the semiconductor layer constituting the two-dimensional radiation detector and the switch interposed between the bias electrode and the bias power source is turned off, based on an output signal of each detection element, i.e. picture element, by the charge amount stored in the condenser during a period from an X-ray irradiation start time t0 to a switch-off time t2 when application of the bias voltage to the bias electrode is shut off, since a charge amount from the X-ray irradiation start time t0 to an X-ray irradiation terminate time t1 is compensated, a picture image signal having an amount corresponding to an incident X-ray amount can be obtained. Thus, there can be obtained the two-dimensional radiation detector having a wide dynamic range wherein the storage amount of the condenser is not saturated, and the switching element connected to the condenser is not subjected to the withstand voltage break.
According to a third aspect of the invention, a two-dimensional radiation detector is structured such that a bias electrode of a semiconductor layer is connected to a bias power source through an impedance element.
According to a fourth aspect of the invention, in the two-dimensional radiation detector of the third aspect of the invention, an electric current or voltage flowing through the impedance element of the impedance element is detected, and a signal read out from each element is compensated based on the detected signal.
With the structure, in the two-dimensional radiation detector of the third aspect of the invention, when radiation, for example, X-ray, enters the semiconductor layer, an electric current flows to the bias electrode of the semiconductor layer from the bias power source through the impedance element. Since the current flowing through the impedance element varies corresponding to an incident X-ray amount, the voltage applied to the bias electrode of the semiconductor layer varies according to the incident X-ray amount. In other words, as the incident X-ray amount becomes larger, a voltage dropping by the impedance element becomes larger, and the voltage to be applied to the bias electrode of the semiconductor layer is lowered by the portion of the dropped voltage.
Therefore, in case a large amount of X-ray enters the semiconductor layer, the voltage applied to the bias electrode of the semiconductor layer is lowered, an electric field in the semiconductor layer is relaxed, and the electron and hole produced through the X-ray incidence are recombined before they reach a signal electrode of the semiconductor layer to increase a rate of annihilating. Therefore, the charges stored in the condenser is suppressed, and the suppression is made corresponding to the incident X-ray amount to the semiconductor layer. Thus, there can be obtained a two-dimensional radiation detector having a wide dynamic range, wherein increase in the voltage of the condenser is suppressed to thereby prevent the storage amount of the condenser from being saturated, and the switching element connected to the condenser is prevented from being subjected to a withstand voltage break.
In the two-dimensional radiation detector of the fourth aspect of the invention, since the storage charge amount suppressed due to falling of the voltage to be applied to the bias electrode of the semiconductor layer is compensated for every picture element, a picture image signal having an amount corresponding to the incident X-ray amount can be obtained and, at the same time, the storage amount of the condenser is prevented from being saturated, and the switching element connected to the condenser is not subjected to the withstand voltage break. Thus, a two-dimensional radiation detector having a wide dynamic range can be obtained.